1. Field of the Invention
The invention relates to an internal combustion engine having two cylinder groups in which a plurality of cylinders are arranged divided into two banks, and upstream sides of exhaust passages of the cylinder groups are connected together by a communicating passage, and as well as to a control method for the internal combustion engine.
2. Description of the Related Art
In a typical V-type multiple cylinder engine, a cylinder block has two banks on its upper portion that are angled at a predetermined angle, with a plurality of cylinders provided in each bank, thereby forming two cylinder groups. Pistons are movably fitted in the plurality of cylinders provided in each bank. The pistons are all connected to a crankshaft that is rotatably supported at a lower portion. Also, combustion chambers are formed by a cylinder head being fastened onto the upper portion of each bank of the cylinder block. An intake port which can be opened and closed by an intake valve, and an exhaust port which can be opened and closed by an exhaust valve, lead into and out of each combustion chamber. An intake pipe is connected to the intake ports of each bank while exhaust pipes are connected to the exhaust ports of each bank. An upstream control catalyst is mounted in these exhaust pipes, and a downstream control catalyst is mounted in an exhaust gas merger pipe into which the two exhaust pipes merge.
In this kind of a V-type multiple cylinder engine, in order to warm up the upstream control catalyst to activate it early on when the engine is started at a low temperature, bank control is made possible by providing communication between the two exhaust pipes with a communicating pipe upstream of the upstream control catalyst and providing a control valve in each exhaust pipe. Accordingly, when the engine is started at a low temperature, the control valve in one of the exhaust pipes is closed, which forces the exhaust gas from the bank on the side with the closed control valve to flow through the communicating pipe and into the other exhaust pipe, where it merges with the exhaust gas from the bank on the other side. The heat from this large amount of exhaust gas efficiently warms the upstream control catalyst, thus enabling it to be activated early on.
Also, with this kind of V-type multiple cylinder engine, a turbocharger is provided for only one of the banks. In this case, when the turbocharger is operating, the control valve in the exhaust pipe without the turbocharger is closed, which forces the exhaust gas from the bank on the side with the closed control valve to flow through the communicating pipe and into the exhaust pipe with the turbocharger, where it merges with the exhaust gas from that side. This large amount of exhaust gas drives a turbine in the turbocharger, which in turn drives a compressor that is integrated with the turbine and compresses air. Introducing this compressed air into the combustion chamber enables a large pressure boost to be obtained as well as suppresses thermal degradation of the upstream control catalyst on the side of the exhaust pipe without the turbocharger.
Moreover, when the downstream control catalyst mounted in the exhaust gas merger pipe is a NOX storage reduction catalyst that stores NOX in the exhaust gas when the air-fuel ratio is lean and releases the stored NOX when the air-fuel ratio is rich, and reduces the released NOX using an added reducing agent (fuel), the NOX purifying efficiency drops when sulfur components in the exhaust are stored. Therefore, controlling the banks such that the exhaust gas from the cylinder group of one bank is lean (i.e., the exhaust gas air-fuel ratio is lean) and the exhaust gas from the cylinder group of the other bank is rich (i.e., the exhaust gas air-fuel ratio is rich) enables sulfur components accumulated in the NOX storage reduction catalyst to be released and the NOX storage reduction catalyst to be recovered using an oxidation exothermic reaction that takes place when the lean exhaust gas and the rich exhaust gas merge just upstream of the NOX storage reduction catalyst.
Japanese Patent Application Publication No. 08-121153 (JP-A-08-121153) describes such an internal combustion engine.
In a V-type multiple cylinder engine, combustion in the cylinders takes place at predetermined intervals and the force generated by this combustion (i.e., engine output) differs depending on the operating state of the engine. Therefore, positive pressure waves of the exhaust gas reach the exhaust pipes that are connected to the cylinder groups of the two banks. A plurality of these positive pressure waves transmitted inside the exhaust pipes generate exhaust gas pulsations inside the communicating pipe. These exhaust gas pulsations that are generated in the communicating pipe prevent the exhaust gas in the combustion chamber from discharging properly into the exhaust pipes through the exhaust ports such that some of the exhaust gas remains in the cylinders. This adversely effects combustion, causes knocking, and adversely effects fuel efficiency and output.
Also, in an engine provided with a turbocharger for one bank, depending on the operating state of the engine, the pressure of the exhaust gas upstream of the turbine may become higher than the pressure (boost pressure) of the intake air downstream of the compressor, which increases the amount of residual gas in the combustion chamber and adversely effects combustion, as described above, so the good capability of the turbocharger cannot be realized. In an engine provided with a NOX storage reduction catalyst, the lean exhaust gas from the cylinder group in one bank ends up merging with the rich exhaust gas from the cylinder group of the other bank in the communicating pipe due to the exhaust gas pulsations generated in the communicating pipe. As a result, not only does an oxidation reaction take place inside the communicating pipe, thus generating heat there, but the oxidation exothermic reaction in the NOX storage reduction catalyst is insufficient which prevents the sulfur components accumulated in the NOX storage reduction catalyst from being properly released.